They built strong walls round the town as a ______ against the e

They built strong walls round the town as a ______ against the enemy.
A、depend
B、defend
C、defeat
D、defense
【正确答案】：D
【题目解析】：depend依靠；defend是动词，保卫，防守；defeat击败；defense名词，防护的意思。句意：他们在城镇周围筑起坚固的城墙以防御敌人。